bt_ctf_clock_value_create(): freeze clock class

bt_ctf_clock_value_create(): freeze clock class

This is how other classes in the library work: when you instantiate a
class as a value (here a clock class as a clock value), the class is
frozen.

This patch does not seem to break the tests or change anything to the
CLI and plugins, which means all clock classes are already configured
before clock values are created out of them.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>